1. Field of the Invention
This invention relates to novel suspended magnetic bearings and more particularly, to bearings which take advantage of the interaction of permanent magnets with high temperature superconducting material.
2. Description of the Prior Art
Conventional mechanical bearings used in conjunction with high rotational speed devices are subject to metal wear, noise, vibration and friction heating problems. These problems can often lead to seizure or other failure of the bearing. In addition, mechanical bearings often require lubricants which fail in severe environments such as those commonly encountered in outer space. Failure of conventional liquid lubricants in outer space is due to the vacuum conditions that cause the lubricants to outgas, leaving bearing surfaces dry and resulting in the ultimate failure of the bearings.
As a result of these and other shortcomings, there has been considerable emphasis on the development of alternatives to mechanical bearings. For example, work has been done to develop more efficient air bearings as well as magnetically suspended bearings.
One problem with air bearings is that they require a completely pneumatic system, including pumps, valves, seals, and conduits, for their operation. Another shortcoming of air bearings is that they result in a continuous energy loss. For example, a high speed cryo-cooler system in outer space applications, would
suffer a 10-20 watt energy loss due to bearing friction losses. Even in non-space applications, use of an air system adds significant cost, size, and weight to the bearing package and introduces various reliability problems normally associated with pneumatic system components.
Air bearings themselves are difficult to manufacture, and thus expensive, because of the fine tolerances required, which are on the order of one ten-thousandth of an inch. Furthermore, air bearings are highly vulnerable to contaminants. A particle of dust as small as four ten-thousandths of an inch can interfere with air gaps and clog pores of graphite or other diffusive coating.
Magnetically suspended bearings have been developed. They are generally unstable and require for their operability control means, such as rapidly acting feedback control systems, to compensate for displacements from the set point. Until recently, magnetic bearings have been of one of two types--either permanent magnets or electromagnets.
The use of permanent magnets is limited to applications where very small forces are adequate. Electromagnets, which can supply considerably more magnetic force than comparable permanent magnets, are much more convenient to use and are thus preferred for use in conjunction with feedback control systems. However, the use of electromagnets adds considerably to the cost, size, and operational complexity of the system.
It has been appreciated for years that magnetic fields strongly interact with superconducting materials. Recent research activities have brought the discovery of "high temperature superconducting" (HTS) compounds. HTS compounds are those which superconduct at and below a critical temperature, T.sub.c, which is above the boiling point temperature of nitrogen.
Since they superconduct at temperatures greater than 77.degree. K., the new CuO high temperature superconductors may be cooled with liquid nitrogen, which is a far less costly refrigerant than helium. As a result, the rather complex thermal insulation and helium-recycling systems, necessary to avoid wasting the expensive helium coolant required for the low temperature superconducting material previously known, is no longer necessary. The YBaCuO HTS compounds simplify and enhance the reliability of commercial applications of superconductors. Liquid nitrogen is about 2000 times more efficient to use in terms of cost, when both the refrigerant itself and the associated refrigerant unit design are considered.
Magnetic fields were disclosed for use as bearings in U.S. Pat. No. 3,810,683. Use of superconductors for support bearings was taught in U.S. Pat. No. 3,378,315, wherein superconducting material is used for a spindle bearing with either permanent magnets or electromagnets providing the supporting magnetic field. U.S. Pat. No. 3,026,151 shows superconductor bearings with the actuator coils likewise formed of superconducting materials.
The recent advances in superconducting materials and the parallel advancements in the field of permanent magnets have made it possible to economically and efficiently couple a superconducting member with a magnetic member to produce highly efficient and relatively inexpensive bearings.
Superconductive materials are of two basic types, designated as Type I and Type II. Efforts have been made in the past to improve magnetic bearing technology by maintaining either the bearing member or the rotating member, or both, in a Type I superconducting state to achieve sufficient magnetic pressure to provide the desired degree of levitation. Unlike Type II superconductors, Type I superconductors are incapable of effecting suspension.
Type I superconductors feature perfect diamagnetism up to a critical applied field, at which point superconductivity is lost and the magnetization of the sample rises abruptly. Examples of superconducting bearings of Type I materials can be found in U.S. Pat. Nos. 3,493,274 and 3,026,151. In order to achieve stability in these systems, the bearing structures must rely on either a mechanical rotary support or must employ superconductors shaped to provide a laterally stable configuration.
The recent discoveries of high temperature superconductors involve Type II materials. Whereas a Type I superconductor completely blocks out magnetic flux from its interior, a phenomenon known as diamagnetism, Type II superconductors allow a certain amount of magnetic flux to penetrate into the interior of the material, producing a suspension effect in addition to a levitation effect. Under such conditions, circulating superconducting currents are established within the superconductor.
A typical example of a system featuring a combination of Type II superconductors and permanent magnets is disclosed in U.S. Pat. No. 4,886,778, which discloses a rotating shaft having two ends, each of which contains a permanent magnet and rotates in a socket clad with superconducting material. The shaft is made to levitate above the sockets by the repulsive forces which exist between the magnets and the superconductors. The incorporation of superconductors into the bearing design offers the possibility of rendering the bearings entirely passive. The design disclosed in U.S. Pat. No. 4,886,778 has the potential for achieving very high rotational speeds, in excess of ten thousand rpm. The interaction between the rotating magnetic axial element and its stationary superconducting support takes place across a gap permeated by a strong magnetic field emanating from permanent magnets embedded in the rotating element.
One difficulty with this and similar configurations lies with the presence of single dipoles which have fixed magnetic fields displaying natural asymmetry. Asymmetry within magnetic field distribution creates drag or dry friction on the rotating member and increases energy losses. Continuous drag is created by the variations of the magnetic field intensity inside the superconducting member, causing energy losses within the superconductor.
The design disclosed in U.S. Pat. No. 4,886,778 features a number of superconducting sockets supporting a permanent dipole magnet through the repulsive forces existing between the permanent magnet and the superconductor. The repulsive force between the magnet and the superconductor depends upon the intensity of the magnetic field as well as the gradient of the field. The gradient is related to the rate of change, in magnitude and direction, of the magnetic field from point to point in the gap between the magnet and the superconductor.
The stronger the magnetic field, the higher the magnetic flux penetration into the superconductor. Although such increased penetration is desirable to achieve higher magnetic stiffness and stability, increased penetration is undesirable in the presence of asymmetry. A combination of asymmetry and high magnetic flux penetration yields increased rotational friction and energy dissipation. Bearing effectiveness is increased by using a magnetic field having a high gradient, and a stronger magnetic field, both of which enhance the magnetic stiffness between the magnet and the superconducting members.
It would be desirable to have a magnetic bearing which would use the repulsive forces generated between a magnet and superconducting material and which would enable a rotating member to reach and sustain ultra-high rotational speeds with low energy dissipation. Specifically, there is a clear need for magnetic bearing systems that produce high magnetic stiffness with minimal magnetic field asymmetry, thus minimizing rotational friction and energy dissipation.
It would be desirable to have a bearing system which would be automatically stable for long periods of time and in all possible directions and which will not require external feedback or other control means to maintain stability.
It would be desirable to have an apparatus for magnetically suspending and centering a body which rotates about an axis.
It would be highly desirable to reduce the asymmetry of a magnetic field being produced by a single dipole.
It would be desirable to be able to generate higher forces and enhanced stability with low energy losses, than would ordinarily be possible with a simple combination of a superconductor and a rotating magnet.
It would be desirable to have a combined bearing system which would include both a gas bearing or an electromagnetic system for purposes of control and also a superconductor/magnet bearing.
It would also be desirable to minimized the dampening in the bearing system by minimizing the magnet asymmetry.
It would be desirable to have an auxiliary control system activated by signals from sensors, such as proximity probes, which are used detect the position of the shaft. The control system would be based on conventional feedback systems such as a gas, foil, tilt pad, or electromagnetic systems and would be activated whenever higher forces are required than those which the magnet is able to produce. The auxiliary system would remain activated until such required forces are again within the operating range of the superconductor and magnet bearing system.
It would be desirable to field cool the magnet to trap the magnetic field lines inside the superconductor and thereby effect high flux penetration as well as magnetic relaxation.